Recently, a passenger-side airbag as well as a driver-side airbag has been provided in a vehicle. Generally, when a vehicle that is provided with a passenger-side airbag including a passenger-side frontal airbag and a passenger-side side airbag is occupied by only a driver without a passenger, in the case of a frontal collision, the passenger-side airbag is not required to be inflated. Accordingly, it is preferable that the passenger-side airbag is only inflated under a condition in which the passenger is resident in the passenger seat.
In order to detect whether the passenger is resident in the passenger seat, a pressure sensor, for example a piezoelectric element, is disposed under the passenger seat such that whether the passenger-side airbag is required to be inflated is determined based on the signal from the pressure sensor.
However, in the non-recommended case that at least two passengers are resident in the passenger seat together, the at least two passengers could be damaged by the inflated airbag. This is especially true for a frontal passenger-side airbag, because the presence of more than one passenger can not be detected.